Sideloading a Modern UI app means to install it without using the Windows Store. This is typically done to beta test an app. This handy guide will teach you how to sideload a Modern UI app on Windows 8. Note that you need a developer license to sideload apps on your PC.

Run the command : Add-AppxPackage (filelocation).appx , substituting (filelocation) with the location of the App's appx package. For example, if your file is located at C:\Apps\example.appx, then run the command:- Add-AppxPackage C:\Apps\example.appx